1. Field of the Invention
The present invention relates to an apparatus for generating negatively charged species using an arc discharge.
2. Statement of the Related Art
FIG. 1 is a schematic diagram of a conventional apparatus for generating negatively charged species which is disclosed in the Journal of Applied Physics, Vol. 61, No. 11, pp. 5,000-5,011, 1 June 1987. In FIG. 1, a cesium reservoir 1 is disposed in a vacuum vessel (not shown), and first and second hollow cathodes 2, 3 are arranged in horizontally symmetrical positions relative to the cesium reservoir 1. A coil having an overall U-shaped configuration is disposed between the first and second hollow cathodes 2, 3. A disk-shaped converter 5 communicating with the cesium reservoir 1 for exchanging charges is provided in a central portion of the coil 4. A disk-shaped anode 6 is provided in a face-to-face relationship with this converter 5. This anode 6 communicates with the inside of a negatively charged ion detector 7 through a central hole 6a thereof. In addition, an ignition electrode 8, opposed to the first hollow cathode 2, has a tip portion thereof placed inside the coil 4.
A description will now be made of the operation of the above-described apparatus for generating a negatively charged species. First, a high voltage is applied between the first hollow cathode 2 and the ignition electrode 8 located at a position 8a indicated by a dotted line, thereby starting a discharge. Subsequently, as the ignition electrode 8 is retracted to the position indicated by the solid line, an arc discharge takes place between the anode 6 and the first hollow cathode 2 to which a high voltage is being applied. In addition, an arc discharge also takes place between the second hollow cathode 3 and the anode 6, and a U-shaped arc discharge is formed between the first hollow cathode 2 and the second hollow cathode 3. Incidentally, after the inside of the vacuum vessel is thoroughly evacuated, hydrogen, argon or the like is admitted to the vacuum vessel until the pressure therein reaches 10.sup.-3 to several mm Hg or thereabouts.
Subsequently, as the cesium reservoir 1 is heated, cesium in the cesium reservoir evaporates, and the evaporated cesium vapor is released through a hole (not shown) formed in the converter 5. Hence, the surface of the converter 5 is covered with the cesium vapor. Furthermore, when hydrogen cations collide with the negative biased converter 5, and hydrogen cations and cesium cations collide with hydrogen atoms adsorbed on the surface of the converter, the hydrogen cations and hydrogen atoms exchange charges with the cesium vapor on the surface of the converter 5 and are thereby converted into hydrogen anions. These hydrogen anions are introduced into the negatively charged ion detector 7 through the hole 6a of the anode 6 and are thereby detected.
Since the conventional apparatus for generating a negatively charged species is arranged as described above, the converter 5 and the cesium reservoir 1 must be provided, and a heater for evaporating cesium must be used. Hence, there have been drawbacks in that the structure is complicated, and the negatively charged species so generated are confined to a gas such as hydrogen or argon.